I've remember that the monster plugin has already included a linux version
you can visit their official site and download the linux version, or visit http://www.meta-mod.com/ if you like.

The method of configuration this plugin is included in the ReadMe file,like

Configuring Metamod to work with plugins:
First, copy the monster_mm.dll file into the same dlls folder where you put
the metamod.dll file (for Linux, copy the monster_mm_i386.so to the same
directory where you copied the metamod_i386.so file).
Then, you will need to create a file called "metamod.ini" in the MODs folder
(the same place where the liblist.gam file is found). You can create this
file with any text editor. You will want it to contain these two lines...
win32 dlls/monster_mm.dll
linux dlls/monster_mm_i386.so
...IMPORTANT!!! Note that both win32 and linux use FORWARD SLASHES '/' when
specifying the plugin filename. Don't use the backslash '\' for Windows.
Now when you start the server, you should see the Metamod banner message
and right after it, you should see the MONSTER banner message, like this...
[MONSTER] Monster v3.00.00, 06/30/2002
[MONSTER] by botman <botman@planethalflife.com>